It's still "right there" overall, you can take a local flight that takes one hour.
Most importantly, you're in the same country whereas in the case of LHR closing the number of airports able to handle widebody long haulers...are essentially all in countries with different customs and visas.
The US has dozens of smaller commercial and even private airports, same for London honestly so this isn't the greatest arguement except it doesn't need to deal with customs.
At least Ireland and the UK are in one visa regime, outside of Schengen. And because there are plenty of flights between Ireland and Schengen countries, all commercial Irish airports should have passport control.
But Dublin airport has about 1/2 the gates of Heathrow...
Most importantly, you're in the same country whereas in the case of LHR closing the number of airports able to handle widebody long haulers...are essentially all in countries with different customs and visas.